FIGS. 7(a) and 7(b) show cross-sectional structures of a GaAs MESFET having a T-shaped refractory metal gate electrode with a Au/TiN/WSi laminated layer structure produced by a conventional process flow. In the figures, reference numeral 1 designates a semi-insulating GaAs substrate on which an n type GaAs active layer 2 having a gate recess 5 of a predetermined depth is formed. In the gate recess 5 a Schottky junction type refractory metal gate 100 having a T-shaped cross section is disposed, and a source electrode 3 and a drain electrode 4 are disposed on the n type GaAs active layer 2 at opposite sides of the gate recess 5.
The refractory metal gate 100 is a laminated layer structure comprising a WSi film 8 1000 .ANG. thick as a lower layer including a stem part 8' of a T-shaped configuration, a TiN film 9 as a plating feeding layer, and an Au plating layer 11 4000 .ANG. thick. The WSi film 8 and the Au plated layer 11 respectively serve as a Schottky metal layer and a low resistance metal layer forming the Schottky junction type gate 100.
The process flow for fabricating the refractory metal gate electrode 100 will be described with reference to FIGS. 6(a)-6(g).
First of all, a gate recess 5 of a predetermined thickness is formed at a position between the source electrode 3 and the drain electrode 4 on the n type GaAs active layer 2 (FIG. 6(a)), and a spacer film comprising SiO 6 about 5000 .ANG. thick is formed on the entire surface (FIG. 6(b)).
A photoresist (not shown) having an opening with a width of about 0.4 .mu.m corresponding to a gate length is formed on the SiO spacer film 6, and a gate contact hole 7 of a depth reaching the surface of the n type GaAs active layer 2 is formed at a predetermined position of the SiO spacer film 6 corresponding to the center of the gate recess 5 by RIE Reactive Ion Etching) employing a gas including fluorine (FIG. 6(c)), and, a WSi film 8 and a TiN film 9 are successively deposited on the entire surface of the SiO spacer film 6 having the gate contact hole 7 by sputtering (FIG. 6(d)).
A resist pattern 10 having an opening 10a of a predetermined size is formed, and an Au plated layer 11 is formed through the opening 10a by Au electrolytic plating employing the TiN film 9 as a plating feeding layer (FIG. 6(e)).
Subsequently, the Au plating resist pattern 10 is removed with an organic solvent, and the TiN layer 9 and the WSi layer 8 are removed by patterning to leave a gate finger configuration by dry etching employing the Au plated layer 11 as a mask (FIG. 6(f)).
Finally, the SiO spacer film 6 is removed employing an HF series etching solution (FIG. 6(g)), thereby forming the Au/TiN/WSi gate electrode 100 having a T-shaped configuration. The step of removing the SiO spacer film 6 with an HF solution is indispensable to reduce the gate parasitic capacitance occurring between the head portion of a T-shaped configuration and the surface of the n type GaAs active layer 2. The reduction in the gate parasitic capacitance is required for enhancing high frequency characteristics.
In the prior art method for producing the refractory metal gate electrode described above, the T-shaped Au/TiN/WSi gate electrode 100 is quite thermally stable at the junction between the WSi film 8 and the n type GaAs active layer 2, and therefore, it has a high reliability. In addition, the refractory metal gate electrode has a T-shaped configuration so as to have a short gate length of about 0.3 .mu.m and has the Au plated layer 11 at its upper part to have a greatly reduced gate resistance it can achieve a superior high frequency characteristics even with a short gate length. In this prior art method of production of a refractory metal gate electrode, however, when forming the WSi film 8 by sputtering, silicon is segregated at the side wall 7a of the gate contact hole 7 having a narrow concave part of a width of less than 1 .mu.m and a depth of more than 0.1 .mu.m. The silicon composition ratio abnormally increases as compared with the flat part and, therefore, when removing the SiO spacer film 6 employing an HF series etching solution, the WSi film 8 formed at the side wall part 7a corresponding to the stem part 8' of the T-shaped gate electrode 100 is easily dissolved in the HF series etching solution. Furthermore, at the interface between the WSi film 8 and the HF series etching solution, the oxidation reaction represented by the following formula occurs. EQU W+3H.sub.2 O.fwdarw.WO.sub.3 +6H.sup.+ +6e.sup.-
Due to this oxidation reaction, electrons are released by tungsten and silicon is dissolved into the HF series etching solution from the WSi film 8. Those electrons are supplied to the reduction reaction occurring at the interface between the Au plated layer 11 and the HF series etching solution through the TiN film 9 and the Au plated layer 11 from the WSi film 8, the reduction reaction being represented by the following formula. EQU H.sup.+ +e.sup.- .fwdarw.1/2.H.sub.2
Thereby, the WSi film 8 and the Au plated layer 11 respectively serve as electrodes, and the above described system comprising HF solution.vertline.WSi.vertline.Au.vertline.HF solution serves as a battery. This battery produces an electrolytic reaction that accelerates the dissolving of the WSi film 8 formed at the above-described side wall 7a into the HF series etching solution.
When the WSi film 8 and the Au film 11 are separately formed on the insulating film, are soaked in HF solution, and the electro-motive force arising between them is measured, an electro-motive force of about 0.33 V is generated, which is approximately equal to the reduction potential of tungsten, showing that an electrolytic reaction is generated by the WSi.vertline.HF solution.vertline.Au system.
The refractory metal gate electrode 100 which causes the above described battery reaction and segregation of silicon in its production process, unfavorably forms locally dissolved portions 12 and 13 at the stem part 8'. In observing the refractory metal gate electrode 100 in cross section by FIB (focused ion beam), it is found that a missing portion 12a due to localized dissolution, or an abnormally etched portion 13a of the WSi film 8 are produced as shown in FIG. 7(b). Such abnormal dissolution of WSi film 8 causes reattaching of the dissolved tungsten to the surface of the recess 5, and loss of the WSi film 8 of the remaining head portion of a T-shaped configuration to the surface of wafer, thereby causing various problems such as deterioration of the device operation characteristics or a reduction in reliability. Furthermore, the tungstic acid which is produced by tungsten being dissolved into the HF series etching solution will cause, if it includes ammonium ion or alkaline metal ion in its solution, precipitation of a tungstate such as (NH.sub.4) 2WO.sub.4, or Na.sub.2 WO.sub.4 in the vicinity of the gate electrode 100 after water washing and drying, thereby resulting in characteristic deterioration and reduction in reliability due to metal contamination.